1. Field of the Invention
The present invention relates to chemical vapor deposition processes. More particularly, the invention relates to a multi-station deposition apparatus and method.
2. Description of the Background Art
As the size of integrated circuit (IC) devices decreases, the deposition techniques used to form very thin films on substrates has become the focus of much interest. To deposit thin films into ultra-high aspect ratio vias and trenches (e.g., aspect ratios on the order of 20:1), atomic layer deposition (ALD) has been used.
An ALD technique deposits a thin film having a thickness of less than 50 Å by alternating the supply of reactant gases and purging gases. Each reactant gas is adsorbed onto the wafer as a monolayer, i.e., a layer being substantially one atom thick. The monolayers of various reactant gas react with one another to form a thin film. A thin film having a high aspect ratio, good uniformity, as well as good electrical and physical properties can be formed using an ALD process. Also, the ALD films have a lower impurity density than those formed by other deposition methods.
ALD generally involves positioning a wafer in a chamber, generating a vacuum in the chamber, and applying certain reactant gases in short bursts (or pulses) to form a thin film upon the substrate. A purge gas may be applied in between reactant gas bursts. Each burst results in the adsorption of a monolayer of gas. The application of gas bursts may be repeated to deposit a thicker film. Once a film of desired thickness is formed, a purge gas is used to remove residual reactant gases from the chamber, the chamber vacuum is released, and the wafer is removed from the chamber.
In one particular example of ALD, a thin tungsten layer may be formed by alternately pulsing silane (SiH4) gas and tungsten hexafluoride (WF6) gas into a chamber. The reaction between the adsorbed gases on the surface of the wafer produces a thin tungsten film. After the thin layer is formed, hydrogen-reduced tungsten hexafluoride can be used to “bulk fill” tungsten onto the nucleation layer. Such an ALD-based process results in very good step coverage of ultra-high aspect ratio trenches and vias.
One method believed to overcome the inherent slowness of an ALD process is a batch process that simultaneously processes many wafers. Some batch processes involve stacks of processing zones having multiple wafers placed in the zones. Within each zone, a laminar flow of reactant gases is supplied over each wafer in the stack. Although effective at simultaneously processing multiple wafers, the stacked zone processing technique has limited throughput.
Therefore, there is a need in the art for a method and apparatus for ALD processing of multiple wafers simultaneously such that wafer throughput is improved.